The system for predicting tyre performance on sand, measuring sand strength with a cone penetrometer and using non-dimensional empirical curves developed by the Waterways Experiment Station (WES) of the U. S. Army was investigated. A series of tyre tests on dry Cresswell sand were carried out and the results were in complete disagreement with the WES system, in both its original and revised forms. It was therefore decided to try to discover the basic soil mechanics of such a system and modify it accordingly. Critical State Soil Mechanics describes two types of soil behaviour, dilating and weakening or compacting and strengthening. It was found that the first of these processes occurred in most situations likely to be found naturally, compaction occurring only in the loosest states obtainable under laboratory conditions. Under dilating conditions sand strength is described by the density, γ (Gamma). and the angle of internal friction, Ø. The angle of friction, for a single sand, was found to vary over a very wide range, depending on the state of compaction and the confining pressure. Density does not vary greatly. The cone penetrometer gradient, G, was found to be related to Ø at a low confining pressure, and the relationship was well described by the theory of Durgunoglu and Mitchell (1975). This lead to the idea that tractive performance would be dependent on Ø, which would be lower the higher the tyre contact pressure. A series of tyre tests on a single tyre on two sands showed clearly that performance depended on both tyre pressure and tyre load. The WES numeric only contains pressure. It was therefore decided to include both parameters by expressing performance by several curves depending on the tyre deflection. Deflection. being expressed as a ratio of tyre diameter rather than tyre section height. The new system was shown to describe all of the WES data better and more logically than their system. The reason why the system cannot describe performance in Yuma and Mortar sands with the same single curve as for Leighton Buzzard and Cresswell remains a mystery.

Driving with tyres not appropriate for the actual conditions can not only lead to accidents related to the tyres, but also cause detrimental effects on the environment via emission of rubber particles if the driving conditions are causing an unexpectedly high amount of tread wear. Estimating tyre performance in an online setting is therefore of interest, and the feasibility to estimate friction performance, velocity performance, and tread wear utilizing available information from the automotive grade sensors is investigated in this thesis. For the friction performance, a trend analysis is performed to investigate the correlation between tyre stiffness and friction potential. Given that there is a correlation, a model is derived based on the trend having a stiffness parameter as an input in order to predict the friction performance. Tendencies for a linear trend is shown, and a linear regression model is fitted to data and is evaluated by calculating a model fit and studying the residuals. Having a model fit of $80\%$, the precision of the expected values stemming from the proposed model is concluded to be fairly low, but still enough to roughly indicate the friction performance in winter conditions. A tread wear model that can estimate the amount of abrasive wear is also derived, and the proposed model only utilizes available information from the automotive grade sensors. Due to the model having a parameter that is assumed to be highly tyre specific, only a relative wear difference can be calculated. The model is evaluated in a simulation environment by its ability to indicate if a tyre is under the influence of a higher wear caused by a higher ambient temperature. The results indicates that the model is insufficient in an online setting and cannot accurately describe the phenomena of softer tyres having a larger amount of wear caused by a high ambient temperature compared to stiffer tyres. Lastly, a double lane change test (ISO 3888-2) is conducted to determine the critical velocity for cornering manoeuvres, which defines the velocity performance. The test was executed for six different sets of tyres, two of each type (winter, all-season, and summer). The approach to estimate the velocity performance in an online setting is analogue to that of the friction performance, and a trend analysis is performed to investigate the correlation between longitudinal tyre stiffness and the critical velocity. The results are rather unexpected and shows no substantial differences in velocity performance, even though the tyre-road grip felt distinctively worse for the softer tyres according to the driver. It is concluded that the bias stemming from the professional driver's skills might have distorted the results, and that another approach might need to be considered in order to estimate this performance.